This invention is directed to an electronic timepiece having a primary battery power source adapted to be charged by a solar battery, and in particular to an electronic timepiece circuit for detecting the state of charge of a primary battery and selectively gating thereto the charging current produced by a solar cell.
Although electronic timepieces utilizing solar batteries for converting solar light incident thereon to a charging current for charging an electronic timepiece power supply have been provided, such solar battery arrangements have been utilized to charge a secondary battery, such as a nickel-cadmium battery. The secondary battery is utilized to drive the timepiece circuitry and is continually recharged by charging current supplied by the solar battery. Nevertheless, such a prior art secondary battery although capable of charging and discharging, is susceptible to internal leakage, resulting in a loss of charge. A further problem with conventional secondary batteries is that space must be left in the container forming same to permit sufficient space for the evolved gas produced during self-discharge. Nevertheless, because of the small size of an electronic wristwatch, the extra space in the container mitigates against providing a small-sized secondary battery for use in the electronic timepiece. Finally, when utilizing prior art secondary batteries within an electronic wristwatch, a damper must be provided in the battery containing portion to increase the volume, and accordingly a lead wire cannot be pressed against the battery to connect same but instead, a lead wire is required to be electrically connected by soldering or the like. Such soldering clearly increases the difficulty in replacing such batteries.